COPENHAGEN - Shares in Danish biotech firm Genmab GMAB.CO fell as much as 14% on Wednesday after it said it was locked in a legal battle with its partner Johnson & Johnson JNJ.N over royalty payments for its key cancer drug.

Genmab, which specialises in the development of antibody therapeutics for cancer treatment, currently receives royalties from Janssen, a Johnson & Johnson company, for sales of its blockbuster Darzalex drug for treatment of multiple myeloma.

The Danish firm said late on Tuesday that a New York arbitration court will determine whether it is entitled to part of Janssen’s royalty payments to Halozyme HALO.O which supplies the enzyme technology for Darzalex injections.

Janssen started reducing royalty payments to Genmab in the second quarter, the Danish firm said.

The arbitration will also decide the duration of Darzalex royalty payments to Genmab, whether they should expire around 2030, when most of the Genmab patents expire, or in 2035 when Janssen’s patents for Darzalex injections expire.

Most analysts assume Genmab will receive royalty payments until 2035. If the arbitration court rules against Genmab on the duration of Darzalex royalties, it would reduce the biotech firm’s market value by a quarter, according to analysts at Jefferies.

“While Genmab intends to vigorously protect its rights under the agreement, the outcome of any arbitration proceeding, as well as its duration, is inherently uncertain,” Genmab said.

The legal battle creates both an overhang and uncertainty for Genmab shares, which will quickly be priced in, Citi analysts said in a note.

Genmab was the worst performer on the European STOXX 600 index, trading 10.8% lower at 2154 Danish crowns each by 0833 GMT.